Field of the Invention
The present invention relates to ion sensitive electronic devices and the manufacture thereat and in particular a manufacturing method compatible with CMOS fabrication processes and ion sensitive devices produced by a method compatible with CMOS fabrication processes.
Description of the Related Art
There is a long felt need for durable electronic sensors that can reliably and accurately measure a concentration of a selected ion or molecule within a solution, such as a concentration of Malic acid or Lactic acid within a fermenting solution of wine. Yet the prior art fails to optimally provide sensors and methods of manufacturing that are compatible complementary metal-oxide semiconductor fabrication techniques.
The prior art does provide ion sensitive field effect transistors (hereinafter, “ISFETs”) and Chemically Sensitive Field Effect Transistors (or, “ChemFETs”) that are intended to reliably and durably generate measurable electrical behaviors that indicate a concentration of a specific target ion or target molecule within a liquid solution. The scope of meaning of the term ISFET as used within the present disclosure includes a Field Effect Transistor that exhibits chemically sensitive electrical qualities, parameters and/or behavior.
In 1970, Bergveld invented a semiconductor device that was chemically sensitive. Much work has been done to commercialize these technologies. A number of companies already sell ISFET sensors such as: D+T Microelectronica, and Sentron Europe By. In addition other companies sell complete ISFET meters such as Hach and Honeywell's DURAFET™. These commercial ISFETs work well, but they are made in non-standard ways. They often use special-purpose materials, such as Tantalum Oxide. In addition when forming the sensing surfaces they have to use a high temperature process which prevents the use of metal layers, which would melt and delaminate the chip. These high temperature prior art techniques generally subject circuit elements to temperatures above 300 degrees Celsius and thereby present challenges in cost-effectively producing sensing surfaces that are specifically sensitive to any molecule or ion other than hydronium. In particular, these prior art high temperature techniques present technical and economic challenges in commercially producing ISFETs that are sensitized to any ion or molecule other than hydronium. Both of these requirements mean that the prior art ISFETs need to be manufactured in a custom fabrication plant. This requirement makes prior art ISFETs expensive to produce, which in turn limits their application.
The prior art further includes applying sol-gel materials and techniques to deposit an ion-specific sensitizing film on a top layer of a gate oxide of a metal-oxide semiconductor field effect transistor. The prior art sensitizing film is preferably selected to be particularly electrically sensitive to a target molecule such that the influence of the gate on the source/drain current of the instant field effect transistor will reliably indicate a concentration of the target molecule within a solution to which the instant field effect transistor is exposed.
The literature of depositing establishing an ion-specific sensitizing film on a metal oxide, wherein the metal oxide is electrically coupled with a gate of a field effect transistor, often refers to the active ingredient of the sensitizing film as an ionophore. It is understood that the term “ionophore” is widely but loosely used in the art to indicate a category of possible seed molecules that create channels in a comprising polymer film and thereby increase an ion-specific sensitivity of the combination of the polymer film and the coupled metal oxide. The origin of the term ionophore limited the scope of the meaning of this term to a lipid-soluble molecule usually synthesized by microorganisms to transport ions across the lipid bilayer of the cell membrane. The use of this term ionophore is thus avoided in the present disclosure and the term “film seed molecule” is meant to designate a molecule or ion that is suitable for positioning within a film or thin layer and upon a top surface of a metal oxide sensing material and that affects the behavior of a gate of a field effect transistor.
In another aspect of the background of the present invention, the sol-gel process is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. Such methods are used primarily for the deposition of materials, including but not limited to metal oxides. Starting from a colloidal solution (or the “sol” state) that acts as the precursor for an integrated network (or the “gel” state) of either discrete particles or network polymers. Typical components of sol-gel mixtures include, but are not limited to, metal alkoxides and metal salts, including but not limited to chlorides, nitrides and acetates, which undergo various forms of hydrolysis and polycondensation reactions in transitioning from sol state into a gel state.
The prior art further provides applications of imprinting molecular polymers wherein selected seed molecules are introduced into a sol-gel mixture prior to placement of the sol-gel mixture as an element of an electronic circuit. A molecularly imprinted polymer (hereinafter, “MIP”) is a polymer that has been processed using the molecular imprinting technique which leaves cavities in polymer matrix with affinity to a chosen “template” molecule. The process usually involves initiating the polymerization of monomers in the presence of a template seed molecule that is extracted afterwards, thus leaving complementary cavities behind. These polymers have affinity for the original molecule and have been used in applications such as chemical separations, catalysis, and molecular sensors.
The term “sol-gel seed molecule” is used within the present disclosure to distinguish molecules introduced into a metal oxide or sol-gel to form a MIP from those materials identified as film seed molecules.
An obvious way to make ISFETs less expensive is to try and make them in a conventional semiconductor fabrication plant. The prior art does include applications of standard CMOS processes to make ISFETs. The early CMOS ISFETS were developed in the 1980s but were not widely accepted commercial applications. Prior art CMOS ISFETs suffer from drift and hysteresis, which makes them difficult to commercialize. Drift causes the output voltage of the ISFET to change over time even in a constant environment. Hysteresis causes the output voltage to not return to the same level when the chemical concentration changes from A to B and then back to A. The hysteresis change in voltage is different than what would be expected by just the drift if the ISFET had been held at concentration A. Both drift and hysteresis are severe in conventional CMOS ISFETs.
Some commercial applications for prior art CMOS ISFETs have been found. Some companies have managed to use CMOS ISFET sensors in high throughput DNA screening. However, prior art CMOS ISFETs of this type primarily detect the binary presence of a genetic DNA base pair, as opposed to the concentration.
While the prior art discloses sol-gel deposition technology to be applicable to ISFET fabrication, the prior art fails to provide (a.) optimal sol-gel deposition techniques that form effective ion sensitive layers at temperatures below 100 degrees Celsius on electrically conductive traces, (b.) sol-gel processes compatible with complementary metal-oxide semiconductor (hereinafter, “CMOS”) fabrication methods, or (c.) CMOS devices having ion detecting films created by sol-gel methods.
It will be appreciated from the foregoing that there is a long-felt need for a transistor based chemical sensor that is less expensive than a conventional ISFET but that has higher performance characteristics than prior art CMOS ISFETs as manufactured in a conventional fabrication operation. Particularly desired would be a low-cost chemically sensitive sensor that (a.) may be more cost-effectively formed on a printed circuit board; or (b.) may be produced by commonly CMOS fabrication facilities, does not show unacceptable hysteresis and is capable of measuring the concentration of target molecules in either simple solutions or complex mixtures.